Efficient operation of boilers and other steam-run equipment requires chemical treatment of feedwater to control corrosion. Corrosion in such systems generally arises as a result of oxygen attack of steel in water supply equipment, pre-boiler systems, boilers, and condensate return lines. Oxygen attack of steel is exacerbated by the unavoidable high temperatures found in boiler equipment. Since acidic conditions also accelerate corrosion, most boiler systems are run in an alkaline environment.
The action of dissolved gases such as oxygen and carbon dioxide are two of the main factors that lead to feedwater system and boiler corrosion. In order to understand the role of dissolved gases in corrosion, one must understand the electrochemical nature of corrosion.
Corrosion processes involve reactions where one species is oxidized EQU M.fwdarw.M.sup.2+ +2e.sup.-
and another is reduced. EQU x+e.sup.- .fwdarw.x.sup.-
In boiler systems the two species involved in the redox chemistry are typically two different metals, a metal and oxygen, or a metal and water. Under most conditions, oxidation of iron occurs. EQU Fe.sup.0 .fwdarw.Fe.sup.2+ +2e.sup.-
A current of electrons then flows from this anodic region to a point where reduction takes place. If oxygen is present, the cathodic reaction is EQU O.sub.2 +H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.-
In the absence of oxygen, water is reduced to hydrogen. EQU 2H.sub.2 O+2e.sup.-.fwdarw.H.sub.2 +2OH.sup.-
Any agent that inhibits either the anodic or cathodic reaction will stop corrosion from occurring. Metal passivation, the formation of a protective oxide film, is one common example of a process that inhibits corrosion by blocking one of the electrochemical reaction pathways.
The severity of oxygen corrosion will depend on the concentration of dissolved oxygen in the water, water pH and temperature. As water temperature increases, corrosion in feed lines, heaters, boiler, steam and return lines made of iron and steel increases.
In most modern boiler systems, dissolved oxygen is handled by first mechanically removing most of the dissolved oxygen and then chemically scavenging the remainder. Mechanical degasification is typically carried out with deaerating heaters, which will reduce oxygen concentrations to the range of 0.005-0.050 mg/L.
Chemical scavenging of the remaining dissolved oxygen is widely accomplished by treating the water with an oxygen scavenger, such as hydrazine or sodium sulfite. See, for example, the Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, Interscience Publishers, Volume 12, pages 734-771 in reference to hydrazine. As explained in Kirk-Othmer, hydrazine efficiently eliminates the residual oxygen by reacting with the oxygen to give water and gaseous nitrogen. In addition, hydrazine is a good metal passivator since it forms and maintains an adherent protective layer of magnetite over iron surfaces.
It is, however, widely recognized that hydrazine is a toxic chemical. Kirk-Othmer reports that it is highly toxic and readily absorbed through the mouth, skin and respiratory system, and that permanent corneal damage may result from contact with the eye. Low doses may cause central nervous system depression and high doses may cause convulsions and other damaging side effects.
Among other approaches to the scavenging of oxygen in boiler systems include: carbohydrazones as disclosed in U.S. Pat. No. 5,258,125; gallic acid as disclosed in U.S. Pat. No. 4,968,438; hydrazones as disclosed in U.S. Pat. No. 5,384,050; carbohydrazide as disclosed in U.S. Pat. No. 4,269,717 and 1,3 dihydroxy acetone as disclosed in U.S. Pat. No. 4,363,734.
Semicarbazone derivatives have been disclosed as additives in lubricating oils in U.S. Pat. No. 2,322,184. However, there is no indication that they would be effective additives to water-based systems.
Semicarbazides have been disclosed for deoxygenating liquids in U.S. Pat. Nos. 4,399,098 and 5,108,624. A "hydrocarbazide" containing urea derivative is proposed as a boiler water treating agent in U.K. Patent Application No. 2,279,347. The functional group is described as C(O)NH.dbd.NH.sub.2 in the specification and C(O)N.dbd.NH.sub.2 in the claims. However, none of these references teach the substituted semicarbazides of the instant invention.
There is still a need for more efficient and less toxic treatment chemicals. Therefore, it is an object of this invention to provide oxygen scavenging treatments which scavenge oxygen and reduce corrosion rates of steel surfaces under typical boiler use conditions while reducing the potential of exposure to hydrazine.